Electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to produce electric power and reaction products. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly ("MEA") comprising a solid polymer electrolyte or ion-exchange membrane disposed between two porous electrically conductive electrode layers. The anode and cathode each comprise electrocatalyst, which is typically disposed at the membrane/electrode layer interface, to induce the desired electrochemical reaction.
At the anode, the fuel moves through the porous anode layer and is oxidized at the electrocatalyst to produce protons and electrons. The protons migrate through the ion exchange membrane towards the cathode. On the other side of the membrane, the oxidant moves through the porous cathode and reacts with the protons at the cathode electrocatalyst. The electrons travel from the anode to the cathode through an external circuit, producing an electrical current.
Electrochemical fuel cells can operate using various reactants. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant may be substantially pure oxygen or a dilute stream such as air containing oxygen.
In some applications, fuel cell systems may operate almost continuously (e.g., certain stationary power applications). However, in other applications, fuel cell systems may be subjected to frequent start and stop cycles and to prolonged storage periods in between (e.g., portable or traction power applications). Further, in colder climates, such fuel cell systems may frequently be subjected to temperatures below freezing. Such systems therefore must tolerate exposure to sub-zero temperatures without degradation. Additionally, the power output capability from fuel cells is typically very limited at temperatures well below the normal operating temperature. Thus, it is also desirable to be able to start up such systems and bring them up to normal operating temperature in a timely, energy efficient manner, and to maintain the temperature within a desirable range during operation.
A conventional approach for starting up a fuel cell is to employ an external power source (e.g., storage battery) or a heater to heat the fuel cell up to a temperature at which fuel cell operation is commenced. However, this requires additional equipment just for start-up purposes and generally requires a net input of energy during start-up. Problems encountered below freezing may simply be avoided by not allowing the fuel cell temperature to go that low. In many applications however, this is not practical. Another approach for low temperature start-up involves operating the fuel cell during start-up and using some of the power and heat generated within to bring the fuel cell up to normal operating temperature. For instance, U.S. Pat. No. 5,798,186 discloses a method for starting up a solid polymer fuel cell stack involving supplying power from the stack to an external load, and then increasing the power drawn and, optionally, the flow rate of the reactant streams while the stack warms up. Another starting method is disclosed in Japanese Patent Publication (Kokai) No. 07-302607, in which the contact resistance between components in the main body of a fuel cell is increased by reducing the pressure applied to the main body of the fuel cell. Internal energy losses are increased and thus the fuel cell temperature can be increased without using an external power source or heater.
A further complication during start-up of fuel cell systems relates to the possible presence of impurities in the reactant streams, particularly the fuel stream. The fuel stream may contain impurities known as electrocatalyst "poisons" which may adsorb or deposit on the anode electrocatalyst and inhibit the desired electrochemical reaction on the anode. The presence of poisons on the electrocatalyst thus results in reduced fuel cell performance. In the absence of countermeasures, the adsorption or deposition of electrocatalyst poisons may be cumulative, so even minute concentrations of poisons in a fuel stream, may for instance, over time, result in a degree of electrocatalyst poisoning which is detrimental to fuel cell performance. Further, the poisons may adsorb or bind more strongly at lower temperatures thereby aggravating the adverse effect on performance at lower temperature.
Reformate streams derived from hydrocarbons or oxygenated hydrocarbons typically contain a high concentration of hydrogen fuel, but typically also contain electrocatalyst poisons such as carbon monoxide. To reduce the effects of anode electrocatalyst poisoning, it is known to pre-treat the fuel supply stream prior to directing it to the fuel cell. For example, pre-treatment methods may employ catalytic or other methods to convert carbon monoxide to carbon dioxide. However, known pre-treatment methods for reformate streams cannot efficiently remove 100% of the carbon monoxide. Even trace amounts less than 10 ppm can eventually result in electrocatalyst poisoning, particularly at low temperatures. Further, during start-up of a reformate-supplied fuel cell system, the reformer and other related apparatus for pre-treatment must themselves also be started up and brought up to a desirable normal operating temperature. Typically, during start-up, the reformer and pre-treatment apparatus are not as effective in providing fuel with a low level of impurity. Thus, the level of poisons in the reformate is typically higher during start-up than it is at normal operating temperature.
It may be possible to remove electrocatalyst poisons by purging the affected electrode with an inert gas such as nitrogen or with a "clean" reactant stream containing substantially no poisons. Where the adsorption of the poison is reversible, an equilibrium process results in some rejuvenation of the electrocatalyst. However, this approach is not as effective against strongly bound adsorbed poisons and can be very slow. Due to the additional difficulties posed by electrocatalyst poisons during system start-up, often reformate is not supplied to a fuel cell system until both the reformer and fuel cell systems are close to the preferred normal operating temperature.